I have two excercises I use to encourage a proper head set. The first is a trot grid. Set up a trot pole to a small cavaletti to another smaller cavaletti. When you trot through it, make sure you give your horse enough of a release for him to stretch out his head!
I also like to set up V poles on my jumps. I set them up on both the front and the back side of a jump. They encourage my horses to stretch out their head and to have a more correct jump all together!
I would also recommend raised trot poles, especially coming into a grid. This will encourage him to drop his head and neck over the trot poles and open up. Doing them on their own will also help him build up some muscle on his top line which will make it easier to stretch over the fence.
Very small oxers (like 60-70cm) with a closed canter. Then increase the foot and width of the oxer every jump. Don't let they jump over speed like doing water. You can add ooles before and after the jump to keep the canter closed after stretching over the jump.
Hey Ginevra! I would say water jumps or wide oxers. Also if you do leg ins and leg outs or shoulder ins/outs that helps stretch out there legs and their jump. Also a ton of flat work! Hope this helps:)